This invention relates to frequency compensation apparatus and more particularly to tuning circuits for telephone lines.
Distributed stray capacitance of twisted pair lines causes an insertion loss, or attenuation, that increases with frequency. In long telephone lines, that is, lines longer than 18,000 feet, this insertion loss adversely affects plain old telephone service (POTS), which operates in the voice frequency (VF) band. The VF band is the frequency range from 300 Hz to 4 kHz. Load coils added at regular intervals in a long line compensate the distributed stray capacitance, thereby flattening the frequency response of the line in the VF band. However, these added load coils increase the insertion loss at frequencies above the VF band. This loss is a problem for services such as asynchronous digital subscriber line (ADSL), which operates using high frequency signals, that is, signals in the frequency range of 20 kHz to 1.1 MHz, that range hereinafter will be referred to as the high frequency band.
An object of the present invention is to provide an improved load coil device.
According to an aspect of the present invention there is provided a load coil device for a two-conductor transmission line comprising: a first winding for connecting in series with one of the two conductors; a second winding for connecting in series with the other of the two conductors; and means for providing a low impedance path for high frequency signals to bypass the first and second windings.
Conveniently, the means comprises a first capacitor connected in parallel with the first winding, and a second capacitor connected in parallel with the second winding.
An advantage of the present invention is that by inserting it into a twisted pair transmission line it effects the line to provide a relatively flat frequency response in the VF band and a decrease in attenuation in the high frequency band compared to prior art load coils. This frequency response is particularly important for providing ADSL type services on a long line when the capability to provide POTS service on that line is to be maintained.